Applications involving process control and equipment functionality are closely linked to temperature. From a thermodynamic standpoint, a change in molecular motion due to heat being added or removed from a system causes changes in temperature. Temperature changes affect various properties differently, which makes heating and cooling control essential aspects of optimized functionality. This goes beyond simply hot, neutral, or cold and instead constitutes quantifying temperature fluctuations with accurate measurements based on how a particular property varies.
Temperature sensors are widely used to quantify temperature fluctuations. This is done by converting thermal energy from an object into electrical signals with either contact sensors or non-contact sensors using one of the two main signal processing methods: equilibrium and predictive. In the equilibrium method, thermal equilibrium is reached between the sensor and the object being measured; therefore, there is no difference in temperature between the two mediums and consequently no temperature gradients. With predictive signal processing, thermal equilibrium is not reached between the sensor and is determined beforehand instead - hence the term predictive.
As mentioned previously, temperature sensors can be either contact or non-contact. Contact temperature sensors are in contact with the object and measure temperature using the predictive method. In other words, these devices assume that the sensor and object are in thermal equilibrium. Examples of contact sensors include:
- Full system thermometers
- Resistance Temperature Detectors (RTDs)
- Thermistors: Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC)
- Bimetallic Thermometers
Non-contact sensors aren’t in contact with the medium being measured and use convection or radiation to measure its surface temperature. The principle behind this technique is based on the infrared ray emitted by the object. This infrared ray is the portion of the electromagnetic spectrum that lies behind visible light and radio waves. By knowing that all bodies with a temperature above 0 degrees Kelvin radiate infrared energy and applying the Stefan-Boltzmann Law for radiated energy, it’s possible to calculate the rate of heat loss by radiant emission.
Examples of non-contact temperature sensors are:
- Fiber-optic temperature sensor
- Pyrometers (optical and radiation)
- Thermal imagers
- Radiation thermometers
Top five industrial sensors
Whether sensing involving conduction, radiation or convection, various options are available for industrial applications. Similarly, interested users can choose from different outputs such as analog, logic or serial. The type chosen depends on the application and the desired temperature range. The top five temperature sensors are resistance temperature detectors (RTDs), thermocouples, optical pyrometers, semiconductor and fiber optic sensors.
Resistance temperature detectors (RTDs)
RTDs are contact sensors that are characterized according to temperature sensing elements and configurations. Platinum, copper and nickel are the most common temperature sensing metals used in RTDs wheras configurations range from thin-film, wire-wound and coiled.
RTDs operate based on the principle that the resistance of the element’s material changes directly with temperature. Platinum, nickel, and copper have a positive temperature coefficient; as temperature increases, the resistance also increases, demonstrating a linear relationship between the two. Therefore, the sensor's material will determine the temperature range where it can be used. The link between material and temperature change is based on the predictable change in resistance accepted internationally as the standard for specific types of metals.
The most common RTD used in industrial settings is the PT100 made with platinum and has a temperature range of -250 degrees Celsius to 850 degrees Celsius. As a passive sensor, RTDs require acceptable current excitation levels that will not alter temperature and introduce an error. A voltage output
Thermocouples are contact temperature measuring devices that consist of two junctions of dissimilar materials welded together. One junction is kept at the reference temperature (cold), while the other is placed at the measuring medium (hot). Temperature differences between both ends of the junction cause an electric current to flow through the thermocouple. The strength of the current is used to determine the temperature of the medium being measured.
The metal alloys used at both junctions vary and range from Iron-Constantan, Platinum-Rhodium, Copper-Constantan, Chromel-Alumel, Tungsten-Rhenium, among others.
Each material junction combination is designated as a specific type of thermocouple according to international standards. For example, junctions joined with Iron-Constantan are designated as Type J thermocouples, while Chromel–Alumel are designated as Type K thermocouples. The most sensitive thermocouples are Type J, while the least sensitive are Type S. Thermocouple sensitivity, temperature range, material type, resistance and life expectancy are deciding factors when selecting a particular type of thermocouple. The steelmaking industry, process plants and manufacturing sectors are examples of sectors where thermocouples are used.
Thermistors are thermally sensitive resistors that exhibit a change in resistance as the temperature varies. These devices can have a positive or negative temperature coefficient. In negative temperature coefficient (NTC) devices, the resistance value decreases as the temperature increases. For positive temperature coefficient (PTC) devices, the resistance increases as temperature increases. NTC-type thermistors are more popular than PTC.
Thermistors are more sensitive to temperature changes than RTDs and thermocouples. As a result, these devices are suited for application where minimal temperature changes are significant variables. Thermistors are not ideal for high-temperature applications due to being fragile and easily damaged in such conditions.
Thermistors exhibit a non-linear change in resistance that can be linearized with shunting with a fixed resistor. They are constructed from solid-semiconductor materials and are rated according to power rating, resistive value and time constant.
Pyrometers are non-contact temperature measuring devices that work using infrared energy. Pyrometers can be optical or use radiation. Optical pyrometers work by comparing the visible light from an object with an electrically heated wire. Radiation pyrometers operate by measuring the infrared or visible light from an object.
Pyrometers have the advantage of not being in contact with an actual surface to measure temperature. Examples of use include monitoring the mains supply unit or measuring the surface temperature of components within specific machines.
Fiber-optic temperature sensor
Fiber-optic temperature sensors are non-contact devices suited for applications where specific activities are likely to interfere with sensor functionality. For example, in industries where radio frequency interference (RFI) and electromagnetic interference (EMI) can affect operations such as explosion-proof areas or critical turbine areas in power plants. Other applications where fiber optic temperature sensors are used include the semiconductor industry, plastics processing, automated welding, which is known to generate large amounts of electric fields.
Different types of fiber optic temperature sensors exist on the market. One type uses gallium arsenide crystal (GaAs) semiconductor by taking advantage of its material properties in blackbody physics. It's possible to determine the wavelengths and temperature that cause an absorption shift due to the semiconducting crystal's predictable characteristics.
Contact a non-contact sensors can be used for different applications depending on the targeted temperature range, metal alloy used to measure temperature sensitivities, environment and cost. From well-known PT100 to more advanced fiber-optic temperature sensors, molecular motion across a medium is being measured with various technologies.
About the author
Janeita Reid is a civil and wind energy engineer, living in Belgium and originally from Jamaica. She is involved in research about aerodynamic airfoil optimization with discrete adjoints. Through technical writing, she hopes to boost renewable energy deployment worldwide in a transparent, cost-effective, and innovative manner.